Fastener For Attachment By Friction Stir Processing

ABSTRACT

A fastener is provided for attachment to a workpiece by a friction stir process. The fastener includes a base having an underside adapted to seat upon the workpiece and a fastener portion attached to the top side of the base. An anchor portion projects from the underside of the base and is adapted to be rotated and advanced at high speed and inserted into the workpiece by friction stirring of the workpiece. The anchor portion is embedded in the workpiece upon cooling of the workpiece. The anchor portion has an indent in the surface thereof enabling the softened stirred material to flow into the indent and thereby mechanically capture the anchor portion within the solidified workpiece.

FIELD OF THE INVENTION

The present invention relates to attaching a fastener such as a screw ora nut to a work piece and more particularly provides a fastener that isembedded into a work piece by friction stir processing and is highlyresistive to axial extraction and rotation relative to the work piece.

BACKGROUND OF THE INVENTION

It is known in the automotive industry and in other applications toattach a fastener onto a work piece. The fastener, such as a screw, anut, a stud, etc., is most conventionally attached by resistanceelectric welding, and the fastener may have small projections that facetoward the metal component so that the electric resistance heating isconcentrated at these projections in order to facilitate the formationof the weld between the fastener and the metal component. Hence theprocess is often referred to as projection welding.

It has also known that a pair of work pieces, such as metal sheets, canbe joined together by a friction stir rivet process in which a rivet isspun at high speed and advanced into the work pieces, thereby causingthe a region of the work pieces surrounding the rivet to be plasticizedand then, upon cooling, the rivet is captured within the cooled metal ofthe work pieces and thereby attaches the work pieces together. Thisfriction stir riveting process is described in U.S. Pat. No. 6,702,535assigned to the same assignee as this invention and herein incorporatedby reference.

SUMMARY OF THE INVENTION

A fastener is provided for attachment to a workpiece by a friction stirprocess. The fastener includes a base having an underside adapted toseat upon the workpiece and a fastener portion attached to the top sideof the base. An anchor portion projects from the underside of the baseand is adapted to be rotated and advanced at high speed and insertedinto the workpiece by friction stirring of the workpiece. The anchorportion is embedded in the workpiece upon cooling of the workpiece. Theanchor portion has an indent in the surface thereof enabling thesoftened stirred material to flow into the indent and therebymechanically capture the anchor portion within the solidified workpiece.

Further areas of applicability of the present invention will becomeapparent from the detailed description provided hereinafter. It shouldbe understood that the detailed description and specific examples, whileindicating exemplary embodiments of the invention, are intended forpurposes of illustration only and are not intended to limit the scope ofthe invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become more fully understood from thedetailed description and the accompanying drawings, wherein:

FIG. 1 is a side elevation view of a fastener according to theinvention;

FIG. 2 is a section view taken through the fastener of FIG. 1 in thedirection of arrows 2-2 of FIG. 1;

FIG. 3 is a view similar to FIG. 1 but showing the fastener embeddedinto a component by friction stir welding;

FIG. 4 shows a side elevation view of second embodiment of theinvention;

FIG. 5 shows the second embodiment of the invention embedded into aworkpiece by friction stir welding;

FIG. 6 shows a side elevation view of a third embodiment of theinvention; and

FIG. 7 shows a bottom view of the third embodiment of FIG. 6;

FIG. 8 shows a side elevation view of a fourth embodiment of theinvention; and

FIG. 9 shows a bottom view of the fourth embodiment of FIG. 8.

DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENTS

The following description of certain embodiments is merely exemplary innature and is not intended to limit the invention, its application, oruses.

Referring to FIG. 1, a fastener 10 is comprised of a base 12, a threadedstud 14, and an anchor 16. The base 12, threaded stud 14 and anchor 16are preferably of one-piece manufacture, such as cold headed ormachined, but may be two or more separate pieces that are joinedtogether.

The base 12 is a hex shape to enable gripping by a hex socket. Thethreaded stud 14 rises from the top surface 20 of the base 12 and theanchor 16 projects downwardly from the bottom surface 22 of the base 12.

The anchor 16 is of a generally solid cylindrical shape, having agenerally circular cross-section, of varying diameter to develop anenlarged diameter foot portion 24 and a smaller diameter waist portion26. The enlarged foot portion 24 has a bottom foot surface 29. Thus asseen in FIG. 1, an indent region 30 is formed in what would otherwise bethe right cylindrical shape of the anchor 16. Also some portion of theouter surface 28 of the anchor 16 has a flat portion 32 as is moreclearly shown in FIG. 2, thereby forming an indent region 34 in whatwould otherwise be a circular cross-section of the small diameterportion.

Referring to FIG. 3, it is seen that the fastener 10 has been attachedto a workpiece 36 by friction stir processing. In friction stirprocessing, the workpiece 36 is locked in a vise or otherwise heldstationary, while the fastener 10 is locked in a chuck and rotated athigh speed about its central axis 38 while being advanced towardengagement with the workpiece 36. The friction heating of the adjoiningfaces of the fastener 10 and the workpiece 36 cause softening of theworkpiece 36. In FIG. 3 the friction stirred region within the workpieceis indicated at 40, it being understood that as the fastener 10 isrotated and advanced into the workpiece 36, the bottom foot surface 29of enlarged diameter of the foot portion 24 will define the outermostdiametrical reach, indicated at 42 of the stirred region. As the footportion 24 passes deeper and deeper into the work piece 36, the softenedmetal of the work piece 36 is in the form of a right cylindrical well ofsoftened material of the workpiece. As the foot portion 24 proceeds, thesoftened material will back fill around the smaller diameter waistportion 26 and against the flat portion 32, thus filling into theindents 30 and 34. During the final stages of insertion, the bottomsurface 22 of the base 12 will act as a lid to force the softened metalof the workpiece 36 to back fill around the anchor 16, and then theexcess material that is displaced by the volume of the anchor 16 isextruded outwardly to the outer diameter of the base 12 where it formsan annular ring of flash 44. This outward extrusion is facilitated bythe friction heating of the material between the bottom surface 22 ofthe base 12 and the top surface of the workpiece 36.

As the metals cool and the workpiece material regains its originalstrength, the joint is obtained by the mechanical interference betweenthe anchor 16 and the stirred region 40 of the workpiece 36. Theattachment between the fastener 10 and the workpiece 36 is made strongby virtue of the mechanical interlock provided by the shape of theanchor portion 16. In particular, the deposit of solidified workpiecematerial into the indent 30 formed by the difference in diameter betweenthe smaller diameter waist portion 26 and the enlarged diameter footportion 24 causes the anchor portion 16 to be effectively embeddedagainst extraction of the anchor 16 in the axial direction. Furthermore,the deposit of solidified workpiece material into the indent 34 formedby the flat wall 32 of the anchor portion 16 effectively locks theanchor portion 16 against rotation by torque applied to the fastener 10.

It will be understood that the softening of the workpiece and insertionof the anchor portion 16 is enabled by the material of the anchorportion 16 having a relatively higher metal point and/or hardness thanthe metal of the workpiece 36. Thus it is the material of the workpiece36 that is subjected to fluid-like motion such that it may be displacedand induced to flow under the combined axial and rotary motion of thefastener 10 during the insertion process. Then upon cessation ofstirring and the associated heat input, the softened material cools,increases in strength and firmly grips the fastener 10 to produce a highstrength joint between fastener 10 and workpiece 36. Furthermore undersome processing conditions the frictional heat developed may besufficient to cause a softening of both the anchor portion 16 andworkpiece 36 so that metallurgical bonding occurs, with the result thatthe fastener 10 is joined to the workpiece by the combined effect ofboth a mechanical embedment of the anchor portion 16 and a metallurgicalbond.

FIG. 4 shows another embodiment of the invention in which fastener 50includes a base 52, threaded stud 54 that rises from the base 52 and ananchor ring 56 that projects downwardly from the underside 58 of thebase 52. The anchor ring 56 is a hollow cylinder, annular in shape, andis symmetrical about a central axis 59. The anchor ring 56 has anenlarged foot portion 60 and a smaller waist portion 62, defining anouter annular indent region 63 and an annular inner indent region 65.

Referring to FIG. 5, the fastener 50 is friction stir processed forattachment to a workpiece 64 by rotating the fastener 50 at a high speedand advancing the anchor ring 56 into the surface of the workpiece 64 tocreate a region 66 of stirred softened metal. Upon cooling, the anchorring 56 is fixedly attached to the workpiece 64 through the mechanicalinterference resulting from the backfilling of the softened metal intothe indents 63 and 65 defined by the smaller diameter waist portion 62.This locks the anchor ring 56 against axial extraction from theworkpiece 64. Excess material that is displaced by the insertion of theanchor ring 56 is extruded from beneath the base 52 and forms a ring offlash 68 around the base 52. It will be understood that the fastener 50does not have an indent in the cross section of the anchor ring 56.However, if desired, fastener 50 may be modified to provide improvedresistance to rotational motion by adding a structural feature such asthe flat 32 of FIG. 1, or by segmenting the anchor ring as will bediscussed below in reference to FIG. 7.

FIGS. 6 and 7 show another embodiment of the invention in which fastener70 includes a nut 72 having an internally threaded hole 74 and anunderside 75. The nut 72 provides a base 76. A hollow cylindricalstructure 77 depends from the base 76 and includes four arcuate shapedanchor segments 78, 80, 82 and 84 that are separated from one another byvoids 88, 90, 92 and 94 and are arrayed equidistant around the centeraxis 93 of the fastener 70. The voids 88, 90, 92 and 94 define indentregions 95, 96, 97, and 98. The fastener 70 will be rotated at highspeed about center axis 93 and advanced toward a workpiece so that theanchor segments 78, 80, 82, and 84 will friction stir and soften theworkpiece. The softened metal will backfill the indents 95, 96, 97 and98. Upon cooling, the softened metal regains its initially hard stateand the solidified material residing within the indents 95, 96, 97, and98 thereby mechanically locks the fastener against rotation by torqueforces applied upon the fastener 70. It will be understood that thefastener 70 does not have an indent in the axial profile of the anchorsegments that would resist axial pull-out of the fastener 70 from theworkpiece. However, if desired, the fastener 70 may be modified forresistance to axial pull-out by implementing the narrow-waisted crosssection shown in FIG. 4 for the arcuate sections 78, 80, 82, and 84.

FIGS. 8 and 9 show yet another embodiment of the invention in whichfastener 100 includes a nut 102 having an internally threaded hole 104and an underside 106. The nut 102 provides a base 108 for a hollowcylinder shaped anchor ring 112 that projects downwardly from theunderside 106. The anchor ring 112 has four enlarged pillar portions116, 118, 120, and 122 that are arrayed equidistant around the peripheryof the anchor ring 112. The fastener 100 will be rotated at high speedaround its center axis and axially advanced toward a workpiece so thatthe anchor ring 112 will friction stir and soften the workpiece. Thefootprint of the softened region of workpiece material will be definedby the pillars 116, 118, 120 and 122 as it is the pillars that definethe outermost and inner most reach of the anchor ring 112. Upon cooling,the molten metal hardens and the fastener 100 is mechanically lockedagainst rotation by the solidified material that has backfilled intoouter indent regions 126, 128, 130, and 132 defined between the pillarsportions 116, 118, 120 and 122 on the outermost side of the anchor ring112. In addition the solidified material back fills into inner indentregions 134, 136, 138, and 140 defined on the inner surface of theanchor ring 112 between the pillar portions 116, 118, 120 and 122. Ifdesired, the anchor ring 112 may be modified to incorporate thenarrow-waisted cross section of FIG. 4 to better resist axial pull-outof the fastener 100.

The foregoing description of the invention is merely exemplary in natureand, thus, variations thereof are intended to be within the scope of theinvention. For example, the fastener may be a rivet, a nut, a threadedscrew, or other type of conventional faster to which an anchor structureis attached for friction stir embedment into a work piece. And, in viewof the forgoing teachings, a person of skill in the art will appreciatea variety of structural shapes for the anchor portion that will providefor highly effective resistance to extraction and resistance to rotationof the fastener from its friction stir welded attachment to the workpiece. And although the drawings show a metal workpiece, the workpiecemay be a plastic material.

1-12. (canceled)
 13. A fastener for attachment to a workpiece,comprising: a base having an underside adapted to seat upon theworkpiece, and a top side; a fastener portion attached to the top sideof the base; an anchor ring of hollow cylindrical shape projecting fromthe underside of the base and adapted to be rotated and advanced at highspeed and inserted into the workpiece upon friction stirring of theworkpiece by the anchor ring, said anchor ring having indented regionsin which the friction stirred material of the workpiece becomes embeddedto thereby anchor the anchor ring in the workpiece upon cooling of theworkpiece; in which the hollow cylindrical shape is defined by aplurality of annular segments having voids therebetween forming indentsin which the friction stirred material of the workpiece becomes embeddedto thereby anchor the anchor ring in the workpiece.
 14. A fastener forattachment to a workpiece, comprising: a base having an undersideadapted to seat upon the workpiece, and a top side; a fastener portionattached to the top side of the base; an anchor ring of hollowcylindrical shape projecting from the underside of the base and adaptedto be rotated and advanced at high speed and inserted into the workpieceupon friction stirring of the workpiece by the anchor ring, said anchorring having indented regions in which the friction stirred material ofthe workpiece becomes embedded to thereby anchor the anchor ring in theworkpiece upon cooling of the workpiece; in which the anchor ring is ahollow circular cylinder and a plurality of thickened axial extendingpillars are arrayed around the hollow cylinder and the pillars definethe indents. 15-19. (canceled)